The invention relates to an ultra-high-frequency partial discharge signal space positioning method, in particular to an ultra-high-frequency partial discharge signal initial moment distinguishing method. The method includes the following steps that a, signal collection is triggered; the triggering level Vi of a certain signal is set, when it is detected that the real-time voltage exceeds the triggering level Vi, a signal collection system is triggered to start to conduct signal collection; b, a triggering threshold value of four-way waveforms is set; when the triggering level Vi serves as the threshold value and the signal collection initial point, the first peak value of the waveforms is searched for backwards, and a first zero crossing point is searched for further backwards to serve as the wavefront moment of signals. According to the ultra-high-frequency partial discharge signal space positioning method, by the adoption of a threshold value method, the four-way waveforms are allowed to be not fully consistent with each other, and the situation accords with the result of field detection of partial discharge signals. The threshold value method is concise in computing, and reprocessing of the waveform signals is not needed, so that the initial moment is distinguished more accurately; furthermore, when the zero crossing point is computed, the zero crossing point is gradually approached by conducting the quadratic interpolation many times, and the accuracy of the zero crossing point moment is improved.